Raunchy frat comedies are as hard to pull off as any other kind because they have to keep surprising the audience, and “The Hangover” does with a bizarre series of uproarious situations with explanations that just about stay within the bounds of plausibility.

Near the beginning, they wake up with no memory of what happened the night before. They’ve lost a tooth and a classic Mercedes but gained a tiger, a police car, a hooker bride, a naked Asian guy with a crowbar and an appointment with Mike Tyson. Also, during the night one of them apparently gave birth.

The locker room humor is dead-on, full of affectionate hostility and caustic R-rated put-downs. Galifianakis insists his shoulder bag isn’t a purse because Indiana Jones carries one. Retort: “So does Joy Behar.” A doctor who is asked the location of a wedding chapel says he knows exactly where it is: “At the corner of ‘Get a Map’ and ‘F – – – Off.’ “

Helms’ routine is a throwback, the hangdog male out of “The Honeymooners,” but he keeps it relatively fresh with the occasional bizarre moment. (I enjoyed the tiger lullaby he plays on the piano to help the big kitty sleep.)

For the schlumpy, weird and often hilarious Galifianakis, this is the role he was born to play. (Almost. Actually, that role would be the lead in “The John Goodman Story.”) As fuzzy and chunky as Joaquin Phoenix on the Letterman show, he inquires, at Caesars Palace, whether Caesar actually lived there. No. He was too busy perfecting his salad. As the three of them try to figure out why there’s a baby in their hotel room, he brags, “I’ve found a baby before.” Where? “Coffee Bean.”

Cooper hasn’t really developed a comedy persona, but he’s a natural leader who makes the most of his many one-liners. Driving a stolen police car, he gets on the bullhorn to call out, “Woman in the leopard dress — you have an amazing rack.”

The leads — limited as they are to the roles of cool guy, dorky guy, dumb guy — each get plenty of funny lines, and though the script is episodic, there won’t be many complaints about that when there are so many imaginative sketch ideas (naked Asian guy hopping out of a trunk, how to drug an angry tiger).

Will the ultimate compliment be paid to “The Hangover”? Will it be watched as a bachelor-party pregame show, with shots of Jagermeister all around? No doubt. If the women in their lives are lucky, guys will pass out watching the movie before they get into trouble.